1. Field of the Invention
The present invention relates to a many-sided reflection prism which reduces a height of an optical system and a slim, compatible optical pickup. More particularly, the present invention is directed toward an optical pickup including a many-sided reflection prism with a plurality of reflection faces to guide a light beam incident through a transmission face in a horizontal direction perpendicular to a height direction, by reducing a size of the light beam in the height direction, and by using a difference in angles between the reflection faces, and to reflect the guided light beam in the height direction by one reflection face of an angle less than 45° with respect to the horizontal direction.
2. Description of the Related Art
In general, the height of an optical pickup depends upon the effective diameter of an objective lens, i.e., the diameter of an incident light beam, the thickness of the objective lens, and the working distance of the objective lens. Therefore, to implement a compact, slim, light-weight optical pickup, there is a need to reduce the size of an objective lens and other optical elements. In particular, an optical pickup for use in optical recording/reproduction apparatuses, e.g., DVD/CD compatible optical disc drives, which are employed to provide portable personal computers with multiple and complex functions, needs to be slimmed down.
Referring to FIG. 1, a conventional optical pickup has an optical structure constructed such that a light beam traveling horizontally from a light source (not shown) is reflected by a reflection mirror 5 having a sloping reflection face 5a of 45° and then focused onto an optical disc 1 by an objective lens 3.
Recording densities are determined by the size of a light spot focused onto the recording surface of the optical disc 1 by the objective lens 3. The shorter the wavelength of light used and the larger the numerical aperture (NA) of the objective lens, the smaller the light spot. Denoting the effective diameter of a light beam incident on the objective lens 3 and the effective focal length according to the effective diameter as EPD and EFL, respectively, and the incident angle of a light beam focused onto the recording surface of the optical disc 1 as θ, NA=sin θ, and θ=Arc tan{EPD/(2EFL)}. Therefore, in order to maximize the effective numerical aperture for the purpose of minimizing the light spot size for a given condition, there is a need for an incident light beam having a diameter corresponding to the effective diameter of the objective lens 3.
The dimensions of a conventional optical pickup having the optical structure as shown in FIG. 1, in particular a thickness h in its height direction, are determined by the diameter of a light beam incident on the objective lens 3, the thickness of the objective lens 3, the thickness of an actuator (not shown) in its height direction for driving the objective lens 3, the size of the reflection mirror 5, and the inclination angle of the reflection mirror 5 with respect to the horizontal direction perpendicular to the height direction.
When there is a need to increase the size of a light beam incident on the objective lens 3 for a desired numerical aperture, the conventional optical pickup having the optical structure described above needs a reflection mirror 5 having a thickness large enough in the height direction to meet the requirement. Therefore, it is difficult to implement a compact-sized slim optical pickup.
Japanese Laid-open Patent Application No. 5-151606 discloses an optical pickup having a structure enabling slim design implementation, unlike when a reflection mirror 5 slanted at 45° is used.
Referring to FIG. 2, the conventional optical pickup disclosed in Japanese Laid-open Patent Application No. 5-151606 includes a light source 2, a collimating lens 4 which collimates a divergent beam emitted from the light source 3, a beam splitter 6 which splits an incident light beam, and a holographic reflection mirror 7 which reflects a horizontally incident light beam toward the objective lens 3.
The holographic reflection mirror 7 has a structure where a hologram 7a is formed on the reflection face of a prism 7b which is slanted at an acute angle no greater than 45° with respect to an incident light beam from the light source 2.
By using diffraction of the hologram 7a as described above, the path of an incident light beam can be perpendicularly changed by a reflection face slanted no greater than 45°. Accordingly, the thickness of an optical pickup in its height direction can be reduced, and its light intensity profile can be made uniform, compared to the case of the reflection mirror 5 slanted at 45° of FIG. 1.
However, the holographic reflection mirror 7 described above causes chromatic aberration due to the diffraction characteristics of the hologram 7a, so it cannot be applied for two wavelengths traveling on the same path. Therefore, the holographic reflection mirror 7 cannot be practically applied to compatible optical pickups using at least two wavelengths.